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MUTATION AND SELECTION BALANCE - pressure will increase the...

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MUTATION AND SELECTION BALANCE In the real world we will generally not find specific evolutionary forces acting alone; there will always be some other force that might counteract a specific force of interest. Our ability to detect these opposing evolutionary forces depends, of course, on the relative strengths of the two (or more) forces. However, it is instructive to examine the conditions where evolutionary forces oppose one another to give us a feel for the complexity of evolutionary processes. Here we will consider a simple case where mutation introduces a deleterious allele into the population and selection tries to eliminate it. As above we define the mutation rate (u) as the mutation rate to the "a" allele. This will tend to increase the frequency of a (i.e., q will increase). In fact, q increases at a rate of u(1-q) ; remember, (1-q) = p, or the frequency of the A allele. This mutation
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Unformatted text preview: pressure will increase the number of alleles which selection can act against. To select against the a allele, we first will assume complete dominance, i.e., that the deleterious effects of the a allele are only observed in the aa homozygote. Under these conditions, the frequency of "a" (q) decreases by selection at a rate of -sq 2 (1-q) , where s is the selection coefficient. We won't derive this for you, but note that the amount of change generated by this selection is a function of the frequency of the aa homozygote (q 2 ) and the frequency of the A allele (1-q). In other words, the amount of change is proportional to the amount of genetic variation in the population, as we showed last lecture. If we put these terms for mutation and selection together, the amount of change in the a allele is : ∆ q = u(1-q) - sq 2 (1-q)...
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